1. Field of the Invention
The present invention relates to an optical pickup apparatus and a reproduction apparatus for irradiating reference light onto a hologram recording medium onto which information is recorded by interference fringes of signal light and the reference light, and also to a reproduction method for reproducing information from the hologram recording medium.
2. Description of the Related Art
As disclosed in Japanese Patent Application Laid-open No. 2007-79438, for example, there is known a hologram recording and reproduction system that records data by forming holograms. In the hologram recording and reproduction system, by generating, during recording, signal light subjected to a spatial light intensity modulation (intensity modulation) corresponding to recording data and reference light imparted with a predetermined optical intensity pattern and irradiating the generated signal light and reference light onto a hologram recording medium, holograms are formed on the recording medium so that data is recorded.
During reproduction, the reference light is irradiated onto the recording medium. By thus irradiating the same reference light as in the recording (having same intensity pattern as that used in recording) onto the holograms formed in accordance with the irradiation of the signal light and the reference light during the recording, diffracted light corresponding to recorded signal light components is obtained. In other words, a reproduced image (reproduction light) corresponding to recording data is obtained. By detecting the thus-obtained reproduction light using an image sensor such as a CCD (Charge Coupled Device) sensor and a CMOS (Complementary Metal Oxide Semiconductor) sensor, the recorded data can be reproduced.
Moreover, as such a hologram recording and reproduction system, there is known a so-called coaxial system in which reference light and signal light are arranged on the same optical axis and irradiated onto a hologram recording medium via a common objective lens.
FIGS. 14 and 15 are diagrams for explaining hologram recording/reproduction in the coaxial system. FIG. 14 schematically shows a recording technique, and FIG. 15 schematically show a reproduction technique.
It should be noted that in FIGS. 14 and 15, a case where a reflection-type hologram recording medium 100 including a reflective film is used is exemplified.
First, in the hologram recording and reproduction system, an SLM (Spatial Light Modulator) 101 is provided for generating signal light and reference light during recording and generating reference light during reproduction as shown in FIGS. 14 and 15. The SLM 101 includes an intensity modulator that performs an optical intensity modulation on incident light in a pixel unit. The intensity modulator can be constituted of, for example, a liquid crystal panel.
During the recording in FIG. 14, by an intensity modulation of the SLM 101, signal light imparted with an intensity pattern corresponding to recording data and reference light imparted with a predetermined intensity pattern are generated. In the coaxial system, a spatial light modulation is performed on incident light so that the signal light and the reference light are arranged on the same optical axis as shown in the figure. At this time, in general, the signal light is arranged on an inner side whereas the reference light is arranged on an outer side as shown in FIG. 14.
The signal light and the reference light generated by the SLM 101 are irradiated onto the hologram recording medium 100 via an objective lens 102. Accordingly, holograms onto which recording data is reflected are formed on the hologram recording medium 100 by interference fringes of the signal light and the reference light. In other words, data recording is performed by forming holograms.
On the other hand, during the reproduction, the SLM 101 generates reference light (intensity pattern of reference light at this time is same as that used in recording) as shown in FIG. 15A. Then, the reference light is irradiated onto the hologram recording medium 100 via the objective lens 102.
By thus irradiating the reference light onto the hologram recording medium 100, diffracted light corresponding to the holograms formed on the hologram recording medium 100 is obtained as shown in FIG. 15B, with the result that a reproduced image (reproduction light) for the recorded data can be obtained. In this case, the reproduced image is guided to an image sensor 103 as reflected light from the hologram recording medium 100 via the objective lens 102 as shown in the figure.
The image sensor 103 receives the reproduced image guided as described above in a pixel unit and obtains an electrical signal corresponding to an amount of the received light for each pixel to thus obtain a detection image with respect to the reproduced image. An image signal thus detected by the image sensor 103 becomes a read-out signal for the recorded data.
It should be noted that as can be seen from FIGS. 14 and 15, in the hologram recording and reproduction system, recording data is recorded/reproduced in a unit of signal light. In other words, in the hologram recording and reproduction system, one hologram (called hologram page) formed by a single interference of signal light and reference light is a minimum unit for recording/reproduction.